UC Irvine UC Irvine Electronic Theses and Dissertations Title The Search for New Resonant Phenomena using Dijet Events at the ATLAS Detector Permalink https://escholarship.org/uc/item/0d87q84g Author Frate, Meghan Publication Date 2017 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA, IRVINE The Search for New Resonant Phenomena using Dijet Events at the ATLAS Detector DISSERTATION submitted in partial satisfaction of the requirements for the degree of DOCTOR OF PHILOSOPHY in Physics by Meghan Frate Dissertation Committee: Professor Daniel Whiteson, Chair Professor Tim Tait Professor Steven Barwick 2017 c 2017 Meghan Frate DEDICATION To Darren ii TABLE OF CONTENTS Page LIST OF FIGURES vi LIST OF TABLES xi ACKNOWLEDGMENTS xii CURRICULUM VITAE xiii ABSTRACT OF THE DISSERTATION xvi 1 Standard Model 1 1.1 Standard Model . 1 1.1.1 Higgs . 3 1.1.2 Electroweak symmetry breaking . 4 1.1.3 Problems with the Standard Model . 5 1.2 New Physics . 7 1.2.1 Quantum Black Holes . 7 1.2.2 Dark Matter Mediator . 8 1.2.3 Compositeness . 8 1.2.4 Heavy Gauge Bosons . 9 1.2.5 Excited Chiral Bosons . 9 1.3 Quantum Chromodynamics . 9 1.3.1 Renormalization . 11 1.3.2 Confinement . 12 1.3.3 Parton Distribution Functions . 14 1.3.4 Feynman diagrams . 15 1.3.5 NLO corrections . 17 1.4 Collision events . 19 1.4.1 Parton Shower . 20 1.4.2 Hadronization . 20 2 The ATLAS Detector 22 2.1 CERN and the LHC . 22 2.1.1 CERN . 22 2.1.2 The Large Hadron Collider . 23 iii 2.1.3 Luminosity . 25 2.2 ATLAS Detector . 25 2.2.1 Detector Layout . 28 2.2.2 Inner detector . 29 2.2.3 Calorimeters . 35 2.2.4 Hadronic Calorimeter . 37 2.2.5 Muon spectrometer . 39 2.3 Trigger . 41 2.4 Monte Carlo . 45 2.4.1 Signal Models . 46 3 Jet calibration 47 3.1 Jet calibration . 47 3.1.1 Jet algorithms . 47 3.1.2 Truth Jets, Track Jets, Calorimeter Jets, Reconstructed Jets . 49 3.1.3 EM+JES calibration . 49 3.1.4 GSC . 54 3.1.5 In-situ technique . 55 3.1.6 Systematic uncertainties . 63 3.1.7 JER . 63 3.1.8 Jet Cleaning . 68 3.1.9 Trigger . 69 4 Resonant Dijet Analysis 71 4.1 Introduction . 71 4.2 Event selection . 71 4.3 Binning . 73 4.4 Background estimation . 74 4.4.1 SWiFt . 74 4.5 Search Phase . 75 4.5.1 BumpHunter .............................. 76 4.5.2 Results . 77 4.6 Background Uncertainties . 79 4.6.1 Fit parameter uncertainty . 79 4.6.2 Fit Choice uncertainty . 81 4.7 Uncertainties . 83 4.7.1 JES . 83 4.7.2 Luminosity . 83 4.7.3 PDF . 84 4.8 Limits . 84 4.8.1 Bayesian Framework . 85 4.8.2 Limit background . 87 4.8.3 Uncertainties . 87 4.8.4 Results . 89 4.9 Gaussian Limits . 89 iv 4.9.1 Jet Energy Folding . 93 4.9.2 Results . 94 5 Gaussian Processes 96 5.1 Gaussian Process Approach . 96 5.1.1 Gaussian Process Formulation . 97 5.1.2 Covariance Structures from Physical Quantities . 99 5.1.3 Implicit covariance in current background models . 100 5.1.4 Kernel construction . 102 5.1.5 Mean function . 103 5.1.6 Incorporating GPs into the statistical procedure . 104 5.1.7 Fitting Procedure . 105 5.2 Performance studies . 105 5.2.1 Background only tests . 107 5.2.2 Background plus signal fits . 109 5.3 Modeling generic localized signals . 113 5.3.1 Look-elsewhere effect . 117 5.4 Conclusion . 118 Bibliography 121 v LIST OF FIGURES Page 1.1 The layout of the Standard Model. On the left are fermions, split into their respective groups of leptons and quarks, both of which have three families. On the right, the gauge bosons, as well as the Higgs boson. Within each box the particle's mass, spin, and charge are listed [3]. 2 1.2 Measurements of strong coupling αS as a function of energy scale Q, with a smooth fit overlaid [26]. 13 1.3 Feynman diagram of a qq scattering event [32]. 17 1.4 Feynman diagrams of next-to-leading order qq scattering events, with the top diagram being an example of initial state radiation and the bottom an example of a loop correction [32]. 18 1.5 Schematic drawing of parton showering and hadronization. The hard scatter is the red disk. Purple disk is secondary hard scatter. Red and purple lines are the corresponding parton shower. Hadronization is in green, with parton groupings to hadrons in light green disks, dark green disks are hadron decays, and yellow is photon radiation [36]. 21 2.1 The LHC layout with all four detectors, ALICE, CMS, LHCb, and ATLAS, as well as acceleration rings [38]. 24 2.2 An overview of the ATLAS detector with the three inner detectors, Pixel detector, semiconductor tracker, and transition radiation tracker, as well as the central LAr electromagnetic calorimeters and tile calorimeters. It also displays the location of the calorimeter end caps or the LAr hadronic end- cap and forward calorimeter, as well as the muon spectrometer and the inner detector solenoid magnet and the muon spectrometer toroid magnet [40]. 26 2.3 Integrated luminosity per day shown in (a) and running total of luminosity shown in (b) for 2017, where (b) is the cumulative sum of (a) [41]. 27 2.4 ATLAS detector coordinate system. The z-axis is along the beam pipe, the x-axis points radially inward to the center of the LHC ring, and the y-axis points directly upwards. In cylindrical coordinates, the azimuthal angle φ is in the x-y plane parallel to the end caps of the detector, and the azimuthal angle θ is in the y-z plane [40]. 28 2.5 Representation of pseudorapidity η in the x-y plane of the detector [42]. 29 vi 2.6 A cross sectional view of the ATLAS. Neutral particles such as neutrons and photons do not leave tracks in the inner detector, while charge particles are bent with the solenoid field. Electromagnetic particles such as electrons and photon deposit their energy in the electromagnetic calorimeter, while hadrons deposit energy in the hadronic calorimeter. Muons are detected in the inner detector due to their charge, but are minimally ionizing through the calorime- ters, and gets detected in the muon spectrometer. Neutrinos escape unde- tected. Image under CERN copyright. 30 2.7 The ATLAS inner detector with barrel and endcap pieces of the pixel detector, semiconductor tracker, and transition radiation tracker [40]. 31 2.8 The Pixel detector module layout, where modules are placed in concentric cycles and pitched at 20 degrees [44]. 32 2.9 All components of the ATLAS inner detector laid out radially (top) and lon- gitudinally (bottom) with radial depth information of each component [46]. 33 2.10 SCT module with silicon strips placed back to back at a tilt of 40mrad, as can be seen in the slight offset of the silicon sensors [46]. 34 2.11 A sketch of the EM and Hadronic calorimeters [49]. ..